e.g. custom-blue would also associate to a form name of "blue" for that sprite.
So if I created a "blue" skin for bulbasaur, I could then create a matching sprite and end up with a structure like this inside the resource pack:
assets/pixelmon/textures/pokemon/custom-blue/bulbasaur.png
assets/pixelmon/textures/sprites/pokemon/001-blue.png